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Handling Missing Data in Output/Figures

In many cases, the dataset to be used for a meta-analysis will contain studies for which insufficient information is available to compute the observed outcomes (e.g., the risk/odds ratios or the raw/standardized mean differences) or for which the values of potentially relevant moderators/covariates are unknown. We can use the dataset for the BCG vaccine meta-analysis (Colditz et al., 1994) as an illustration.

Suppose the 2×2 table data for the 6th study were not available and let's make the absolute latitude value for the 8th study missing:

As we can see, the 6th study has been omitted from the forest plot. However, we may still want to have the study included in the forest plot (e.g., as a visual cue that the study did in fact exist). This is possible by changing the na.action option under the global options (see help(options) for more details on options settings). In particular, the default setting in R for na.action is na.omit, which results in the behavior shown above.

And now, the 6th study is shown in the plot (of course, since the log risk ratio and corresponding CI cannot be computed, the actual results remain missing).

When inspecting output, this issue may also be relevant. For example, suppose we fit a mixed-effects meta-regression model to the log risk ratios using the absolute latitude of the study location as a potential moderator: